1. Field of the Invention
The present invention relates to the field of measuring and manufacturing optical surfaces. In particular the invention relates to a method of calibrating an interferometer apparatus for measuring an optical surface and/or a method of processing the optical surface by using the interferometer apparatus.
2. Brief Description of Related Art
An optical substrate having the optical surface can be, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, or systems used for imaging structures, such as structures of a mask or reticle onto a radiation sensitive substrate, such as a resist in a lithographic process. The success of such an optical system is substantially determined by the precision with which the optical surface can be machined or manufactured to have a target shape. In such manufacture, it is necessary to compare the shape of the machined optical surface with its target shape, and to determine differences between the machined and target surfaces. The optical surface is then further machined at those areas where the differences between the machined and target surfaces exceed e.g. a predefined threshold.
In order to precisely measure the shape of an optical surface, an interferometer apparatus is commonly used. Examples of conventional interferometer apparatuses such as a Fizeau interferometer or a Twyman-Green interferometer are given in D. Malacara: “Optical shop testing”, 2nd edition, John Wiley & Sons Inc, 1992, pages 1 to 172. Further, examples of such interferometers are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477, which documents are incorporated herein by reference.
The conventional interferometer apparatus usually includes a reference surface which is illuminated with measuring light. Measuring light reflected back from the reference surface is directed onto a camera having a plurality of detector elements. Further, the optical surface to be measured is arranged in a same or a separate beam of measuring light, and the optical surface is also imaged onto the camera by using light interacting with the optical surface to be measured. The light interacting with the optical surface and light reflected from the reference surface generate an interference pattern on the camera. By analyzing this pattern, shape differences between the target surface and the optical surface to be measured can be determined in terms of wavelengths of the measuring light at respective locations on the optical surface. Examples for methods for measuring shape errors of an optical surface by using an interferometer apparatus are e.g. disclosed in U.S. Pat. No. 4,697,927 A1 and U.S. Pat. No. 6,456,382 B2, the contents of which are incorporated herein by reference.
For determining the shape differences at the respective locations on the optical surface, it is necessary to establish a correspondence between lateral coordinate positions of the respective detector elements of the camera and corresponding lateral coordinate positions of the respective locations on the optical surface. This correspondence is determined by the imaging properties of an interferometer optics performing the imaging from the optical surface onto the detector.
US 2002/0080366 A1 discloses a method for determining imaging properties of interferometer optics in connection with establishing correspondences between lateral coordinate positions of respective detector elements and respective locations on an optical surface. This method includes a calibrating step in which a calibrating element is positioned in a beam path of the interferometer apparatus at a location where the optical element to be measured will be positioned during a measuring step. The calibration element is designed such that it has substantially a same effect on the measuring light as the optical element to be measured, with the exception that a plurality of circular apertures is provided in the calibrating element at predetermined positions. These apertures are imaged onto the camera by the interferometer optics, and an image of the apertures taken by the camera is analyzed for determining imaging properties of the interferometer optics, such as a distortion and a scale factor.
It has been noted that the correspondence between lateral coordinate positions on the optical surface and on the camera determined by the conventional method is limited.